EXPLANATION OF THE PLATES. 281 



PLATE V. AI/TERNATION IN THE ENTOZOA, (foe. 



Figs. 1-3 illustrate the successive phases in the development 

 of the Tapeworm (pp. 96, 131, 161). 



Fig. 1. The egg of a species of Tapeworm, Tcenia Pistillum, 

 (from Dujardin), with its six-hooked contractile embryo. 



Fig. 2. The Cysticerus (from Bremser), now known to be the 

 cystic phase of the Tcenia Solium ; a the head, formed originally 

 within the cyst 6, and then extruded by the evagination of its 

 hollow pedicle or neck. In the farther development within the 

 alimentary canal, the cyst is thrown off, and the neck extended, 

 by the gemmation and multiplication of its joints, into the long 

 body of the Tapeworm. 



Fig. 3. Ultimate development of the Tcenia Pistillum of the 

 Shrew (from Dujardin) a, the hea ; 6, the neck, formed of un- 

 developed segments ; c, the body, formed of segments which 

 have acquired their full size, and become filled with ova. 



Figs. 4-7 illustrate the development of the JEchinococcus or 

 Hydatid (p. 131). 



Fig. 4. A diagram of the simple Hydatid cyst (EcMnococcus 

 scolicipariens) the first result of the transformation of the con- 

 tractile vesicle discharged from the egg of a minute species of 

 Tcenia. Derivative Taenia-buds are represented in the course of 

 development from the interior of the cyst (p. 131). 



Fig. 5. A diagram of the compound Hydatid (Echinococcus 

 altricipariens), resulting from the intermediate gemmation of 

 secondary cysts, prior to the formation of proper Taenia-buds. 

 Fig. 6. A Tsenia-bud, with its denticulated head a in process 

 of development, on an invagination of the wall of the cystic por- 

 tion 6 ; c pedicle of attachment to the parent cyst. 



Fig. 7. A fully-formed Tsenia-bud, with its head everted 

 (both from Wilson). 



Figs. 8-11 (mostly from Steenstrup) illustrate the successive 

 forms in the alternation of the Trematoda (pp. 95, 112). 



Fig. 8. Egg of Monostomum mutaUk (from Siebold) with its 

 infusorian embryo. 



Fig. 9. Redia or intermediate form of Distoma, derived from 

 the infusoriao, and forming in its own interior an agamic brood of 



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